The present invention relates generally to the field of capacitors. More specifically, the present invention relates to electrolytic capacitors for use in a device such as an implantable medical device.
Since their earliest inception, there has been significant advancement in the field of body-implantable electronic medical devices. Today, such implantable devices include therapeutic and diagnostic devices, such as pacemakers, cardioverters, defibrillators, neural stimulators, drug administering devices, and the like for alleviating the adverse effects of various health ailments.
Implantable medical devices may utilize a capacitor to perform various functions. For example, if the implantable medical device is a defibrillator, one or more capacitors may be used to provide a therapeutic high voltage treatment to a patient.
One type of capacitor that may be used in such an application is an electrolytic or wet slug capacitor. Conventional wet slug capacitors may include a container formed from tantalum or a tantalum alloy that acts as the cathode. An electrolyte (e.g., an acid such as phosphoric acid) and an anode are provided within the container. In these types of capacitors, an anodic oxide may be formed on exposed surfaces.
Since the electrolyte is electrically conductive, a conductor-insulator-conductor structure including metal, oxide coating, and electrolyte is present at both the anode and the cathode. Each of these conductor-insulator-conductor structures constitute themselves a capacitor.
In the conventional wet slug capacitor, the anode capacitance is electrically connected in series with the cathode capacitance. The total capacitance Ctotal of the two capacitors Canode and Ccathode in series is expressed by the formula 1/Ctotal=1/Canode+1/Ccathode. In order to maximize Ctotal, the capacitance Ccathode has to be as large as possible.
Although conventional wet slug capacitors having useful capacitances have been produced, there is a desire to increase the energy per unit volume of capacitor anode (i.e., the stored energy density). The energy E stored inside a capacitor may be expressed by the formula E=½ CtotalU2, where U is the potential to which the capacitor is charged. Hence, high energy density requirements demand high-capacitance per-unit area cathodes so as to maximize Ctotal and, in turn, E. Conventional capacitor cathode materials (e.g., tantalum), however, may provide a limited capacitance per unit area. For certain applications, it is desirable to provide a capacitor cathode that has a capacitance in the range of approximately 10–20 milliFarads per square centimeter of geometrical surface area.
Accordingly, there is a need to provide an electrode for a capacitor that utilizes a material which enhances the capacitance for the electrode relative to conventional capacitor electrodes (e.g., provides a capacitor electrode having a specific capacitance of greater than approximately 10 milliFarads per square centimeter). It would be desirable to provide a method of producing such an electrode using a method which is relatively simple in terms of the processing involved and that does not adversely affect capacitor performance. There is also a need for an electrode that utilizes a material which produces a relatively smooth and defect-free electrode surface. There is further a need to provide a capacitor that includes at least one electrode that exhibits increased capacitance as compared to conventional capacitor electrodes.
It would be desirable to provide an electrode for a capacitor and a capacitor that provides one or more of these or other features. Other features will be made apparent from the present description. The teachings disclosed extend to those embodiments that fall within the scope of the appended claims, regardless of whether they provide one or more of the aforementioned features.